Sunday, December 18, 2022

ECG Blog #351 — Posterior Leads ...

 — See ECG Blog #406 — for a Video presentation of this case!

The ECG in Figure-1 — was obtained from a previously healthy older man who contacted EMS (Emergency Medical Services) because of "chest tightness" that began ~1 hour earlier. Given this history:

  • How would YOU interpret the ECG in Figure-1?
  • Should you activate the cath lab ?

Figure-1: The initial ECG in today's case — obtained by EMS at the scene, from an older man with ~1-hour of chest "tightness". (To improve visualization — I've digitized the original ECG using PMcardio).

MY Thoughts on the ECG in Figure-1:

The rhythm in ECG #1 is sinus at ~75/minute. Regarding intervals — the PR interval is normal — the QRS is not wide — and the QTc is at most borderline prolonged. The frontal plane axis is normal at about +70 degrees. There is no chamber enlargement.

Regarding Q-R-S-T Changes:

  • A large and wide Q wave is seen in lead aVL. There are no other Q waves.

  • R wave progression is appropriate — with transition (where the R wave becomes taller than the S wave is deep) occurring normally between leads V3-to-V4.

Regarding ST-T Wave Changes:
  • PEARL #1: The most noteworthy findings in today's tracing are in lead V4. The downward sloping "shelf-like" shape of the ST segment in this lead, in association with terminal T wave positivity in a patient with new-onset chest pain — is almost by itself diagnostic of acute posterior OMI (Occlusion-based Myocardial Infarction).

  • In strong support of my impression from lead V4 — is the shape of neighboring lead V3. Although there is somewhat less ST depression in lead V3 (and its ST segment is now gently upsloping) — there now is significantly more positivity of the terminal T wave.
  • The other "neighboring lead" to V4 is lead V5. While not nearly as abnormal as leads V3,V4 — lead V5 shows subtle-but-real "scooped" ST depression. 

  • PEARL #2: In a patient with new chest pain — the finding of ST depression that is maximal between leads V2-to-V4 should suggest acute posterior OMI until proven otherwise. This provides an important distinction between diffuse SEI (SubEndocardial Ischemia) — in which near comparable ST depression is seen in multiple leads, with ST elevation in lead aVR (See ECG Blog #271).

  • PEARL #3 (Beyond-the-Core): There is a subtle-but-real abnormality in lead V2 — in that the ST segment in lead V2 is flat (ie, neither elevated nor depressed). This is unusual — because much (most) of the time in a normal tracing, there is slight upsloping ST elevation in lead V2. The finding of a flat ST segment in lead V2 — is most likely the result of an "attenuating" (opposing) effect from acute posterior OMI that produces some ST depression.

  • The ST segment in lead V1 is coved and slightly elevated. The significance of this finding is uncertain from this tracing.
  • Lead V6 is unremarkable.

  • In the Limb Leads — There is subtle-but-real "scooped" ST depression with terminal T wave positivity in each of the inferior leads.
  • The subtle coved shape of the ST segment in lead aVL (with slight ST elevation and suggestion of terminal T wave inversion) — appears to be a reciprocal change to the ST-T wave appearance in the inferior leads.
  • I thought lead I was nondiagnostic. 

ECG #1 — Putting It All Together:
As alluded to earlier — in an older patient with new-onset chest pain — the finding of maximal ST depression in leads V3 and V4, with subtle-but-real findings in neighboring leads — is diagnostic of acute posterior OMI until proven otherwise. To Emphasize: In a patient with new chest pain — this single ECG by itself should suffice to merit prompt cath lab activation!
  • Although impossible to tell from this single tracing if the large and wide Q wave in lead aVL is new or old — the associated ST coving with slight elevation and hint of terminal T wave inversion — suggests acute high-lateral involvement. This is supported by the finding of reciprocal ST depression in each of the inferior leads.
  • Regarding prediction of the "culprit" artery — The finding of suspected acute posterior and high-lateral OMI in the absence of ST elevation in the inferior leads suggests the LCx (Left Circumflex) as the acutely occluded artery (especially of the 1st or 2nd Obtuse Marginal Branch).

  • BOTTOM Line: Regardless of what turns out to be the "culprit" artery — the important point is that in a patient with new chest pain — ECG #1 by itself is diagnostic of acute OMI until proven otherwise. The cath lab should be activated.

PEARL #4: Use of the "Mirror" Test
I favor use of the "Mirror Test" — as a visual aid to facilitate recognition of acute posterior MI. This adds even more support to the above "Bottom-Line conclusion", that as I emphasize above, should have already been reached.
  • The principle of this test is simple: It is based on the fact that the mirror-image view of anterior leads — provides insight to the nature of electrical activity as viewed by the posterior wall of the left ventricle (For more on the "Mirror" TestSee the Audio Pearl in the ADDENDUM below).

  • Note that I have vertically flipped anterior leads V2 and V3 in the PURPLE inserts that I've added in Figure-2 (to show the mirror-image view of these 2 leads). Doesn't the shape of the "shelf-like" ST depression and terminal T wave positivity seen in leads V3 and V4 of Figure-2, when vertically flipped (as viewed in the Mirror Test) suggest a worrisome shape and amount of ST elevation — as well as already deep T wave inversion?

Figure-2: I've added the mirror-image of leads V3 and V4 to today's tracing — to illustrate how the initial ECG shows a positive "Mirror" Test suggestive of acute posterior OMI (See text).

What Did the Repeat ECG Show?
The EMS team immediately recognized the need to urge cath lab activation and to expedite transport of the patient to the hospital. 
  • En route — the EMS team obtained a 2nd ECG (shown in Figure-3) — with substitution of right-sided lead V4R and posterior leads V8 and V9 in place of leads V4,V5,V6.

  • How would YOU interpret ECG #2— considering that it was obtained 18 minutes after ECG #1 (with the patient describing a reduction in chest pain severity).

  • What are potential Pros & Cons of substituting leads V4R, V8, V9 for V4,V5,V6?

Figure-3: Comparison of the initial ECG — with a repeat ECG done 18 minutes later (substituting leads V4R,V8,V9 — for leads V4,V5,V6). Of note — the patient reported a reduction in chest pain severity around the time ECG #2 was recorded.

Potential Pros & Cons of Additional Leads:
I will never negate the potential benefit of recording additional leads when the purpose for doing so is: i) To increase the confidence of the treating provider in his/her interpretation; — or — ii) Because the treating provider(s) feel the need to "convince" the on-call cardiologist that posterior leads show the ST elevation that is being looked for to "justify" taking the patient to the cath lab. That said — the clinical realities are the following:
  • It takes an extra moment of time to record additional leads. As a result — EMS providers must "balance" their need to expedite transport to the ED — vs — spending that extra moment of time because your on-call cardiologist is one who will not activate the cath lab without ST elevation.

  • The reality is that there is no ST elevation on a standard 12 lead ECG with isolated posterior infarction. That said, as is the case for today's patient — ECG #1 is diagnostic by itself (without the need for ST elevation) — of acute coronary occlusion (ie, of acute OMI = Occlusion-based Myocardial Infarction).

  • In the April 1, 2018 post in Dr. Smith’s ECG Blog — Drs. Meyers, Weingart and Smith published their OMI Manifesto — in which they extensively document the critically important concept that management of acute MI by separation into a “STEMI” vs “non-STEMI” classification is an irreversibly flawed approach.
  • Their OMI Manifesto details how use of standard STEMI criteria results in an unacceptable level of inaccuracy, in which an estimated 25-30% of acute coronary occlusions are missed! Yet despite this remarkable flaw in the STEMI-paradigm — a substantial number (if not a frank majority) of clinicians continue to apply outdated criteria when interpreting ECGs, by refusing to consider prompt cath for definitive diagnosis and reperfusion therapy just because a millimeter-based definition for acute STEMI is not satisfied.
  • The September 3, 2020 post of Dr. Smith's ECG Blog features Dr. Meyers' 17-minute summary of this OMI Manifesto, with all of its documentation.

  • I summarize KEY points from the OMI Manifesto in the July 31, 2020 post in Dr. Smith's ECG Blog. For clarity, in Figure-4 (below in the ADDENDUM) — I've put the ECG findings to look for that suggest acute OMI — despite the fact that your patient with new chest pain does not satisfy sufficient criteria to qualify as a "STEMI". 

Regarding Posterior Leads:
While many providers (including many cardiologists) favor the use of posterior leads — the reality is that QRST amplitudes with posterior leads are reduced compared to mirror-image anterior lead amplitudes. This is because to assess electrical activity from posteriorly placed V7,V8,V9 electrodes — the thick back musculature (with its significant damping effect) must be traversed before these posterior leads are able to record the heart's electrical activity.
  • To Emphasize: I am not against those who prefer to obtain posterior leads because they feel this helps in their interpretation. I am simply saying that with minimal practice using the Mirror Test (See Pearl #4 and Figure-2 above) — that equal information is obtained faster without the need to apply additional leads. (Actually — more information is obtained — since there are times when the Mirror Test is positive despite negative posterior leads).

Regarding Right-Sided Leads:
As discussed in ECG Blog #190 — the BEST way to diagnose acute RV MI is by seeing ST elevation in right-sided leads. The standard 12-lead ECG may hint at acute RV involvement (ie, If in the face of ST depression in leads V2,V3 from acute posterior MI — there is unexpected ST elevation in lead V1). But by itself — the standard 12-lead ECG cannot rule out the possibility of acute RV involvement
  • That said — Since acute RV MI is virtually always caused by acute RCA (Right Coronary Artery) occlusion — there should (by definition) — be ST elevation in the inferior leads in patients who also have acute RV involvement. The complete lack of such ST elevation in the inferior leads of ECG #1 — all but negates the possibility of acute RV MI in today's case (ie, isolated RV MI is rare!).

What do We Learn from ECG #2?
The reason I have put the 2 ECGs in today's case together in Figure-3 (shown above) — is that doing so makes it much easier to apply lead-to-lead comparison.
  • In the Limb Leads The frontal plane axis is similar in both ECG #1 and ECG #2. This tells us that lead-to-lead comparison in the limb leads will be valid!
  • The heart rate in ECG #2 is a little bit slower.
  • There appears to be slightly less ST depression in each of the inferior leads.

  • In the Chest Leads — We can only compare leads V1,V2,V3, since leads V4R,V8,V9 have replaced lateral chest leads V4,V5,V6 in ECG #2. That said — both the amount of J-point ST depression and the height of the hyperacute T wave in lead V3 are clearly less in ECG #2!

  • IMPRESSION: In view of the patient's report that chest pain was decreasing at around the time that ECG #2 was recorded — this improvement in ST-T wave appearance suggests that the "culprit" vessel may have been in process of spontaneously reopening.

PEARL #5: Often overlooked (but of critical clinical importance) — is the need for close correlation between serial ECGs and the relative severity of chest pain symptoms at the time that each ECG is obtained. 
  • The reason this close correlation is so important — is that it provides KEY insight as to whether the acutely occluded artery has spontaneously reopenedor — is likely to still be occluded. 

  • KEY Point: The process of acute coronary occlusion is often not "all or none". Instead — many patients have a "stuttering pattern" — in which the acutely occluded artery may spontaneously reopen, only to later spontaneously reocclude. And sometimes, the pattern is of spontaneous opening — followed by reclosure — then reopening — another reclosure — with this process going back-and-forth occurring multiple times — until eventually at some point, a "final" state of either open or closed is reached for the "culprit" vessel.
  • Persistent ST elevation with ongoing severe chest pain suggests that the "culprit" artery is still occluded. 
  • In contrast — Reduced chest pain that occurs as ST elevation is resolving (and as reperfusion T waves develop) — suggests that the "culprit" artery has spontaneously opened (See the Audio Pearl in today's ADDENDUM below for more on this).

PEARL #6: If the decision is made to obtain additional leads when it comes time to repeat the ECG — Be SURE to obtain a 2nd complete 12-lead ECG before you record right-sided or posterior leads! Otherwise — you lose data.
  • Ironically in today's case — If the purpose for obtaining posterior leads in ECG #2 was in the hope of convincing the on-call cardiologist that there is ST elevation in leads V8,V9 — the fact that chest pain was decreasing and acute ST-T wave changes in the inferior leads and in lead V3 seemed to be resolving — will probably also result in less ST elevation in leads V8,V9.
  • It probably would have been much easier to establish that acute ST-T wave changes were resolving in ECG #2 if a standard lead V4 would have been recorded in this repeat ECG.

  • Recording a complete 12-lead ECG before obtaining additional leads would have accomplished 2 things: i) Documenting a change in ST-T wave findings on serial ECGs in a patient with changing severity of symptoms is the definition of "dynamic" ST-T wave change — and, in a patient with new chest pain, this finding alone is indication for prompt cath!and, ii) Seeing reduced acute ST-T wave changes in a standard lead V4 in ECG #2 — would have better prepared us not to expect much (if any) posterior lead ST elevation.

  • It turns out that leads V8 and V9 in ECG #2 do show ST coving and slight elevation — but this is minimal.

CASE Follow-Up:
Today's case provides an excellent example of superb care by the EMS team — that quickly recognized acute OMI — and — quickly recognized the need for prompt cath. The on-call cardiologist did perform emergent cardiac catheterization because of persistent chest pain and abnormal ECG changes (I do not have access to subsequent serial tracings on this patient).
  • Cardiac cath showed multi-vessel disease — with acute occlusion of the 1st OM (Obtuse Marginal branch) of the LCx (Left Circumflex) artery felt to be the acute "culprit" lesion. A drug-eluting stent was placed — and the patient apparently did well!


Acknowledgment: My appreciation to Tomasz Numrych (from Maryland, USA) for the case and this tracing.


ADDENDUM (12/18/2022): 

  • Included below are a series of links and other material relevant to detection of the “culprit” artery — and my thoughts for making the case to replace the term “STEMI” with “OMI”, in the hope of substantially increasing detection of acute coronary occlusion. 


Free PDF Downloads from relevant Sections in my ECG-2014-ePub:

  • PDF File: Overview on the Cardiac Circulation and the “Culprit” Artery in Acute MI —
  • PDF File: Posterior MI and the “Mirror Test” —


Figure-4: ECG findings to look for when your patient with new-onset cardiac symptoms does not manifest STEMI-criteria ST elevation on ECG. For more on this subject — SEE the September 3, 2020 post in Dr. Smith’s ECG Blog with 20-minute video talk by Dr. Meyers on The OMI Manifesto. For my clarifying Figure illustrating T-QRS-D (2nd bullet) — See My Comment at the bottom of the page in Dr. Smith’s November 14, 2019 post.


Figure-5: KEY points in the recognition of isolated posterior MI (This figure is taken from ECG Blog #193 — in which I review the "Basics" for predicting the "culprit" artery)

ECG Media PEARL #60 (8:30 minutes Audio) — Reviews use of the "Mirror Test" to facilitate recognition of: i) Acute Posterior MI; ii) Acute High-Lateral or Inferior MI (ie, the "magical" reciprocal relationship between leads III and aVL)andiii) Anterior ST elevation due to LVH (that is not indicative of anterior MI).


ECG Media PEARL #10 (10 minutes Audio) — reviews the concept of why the term “OMI” ( = Occlusion-based MIshould replace the more familiar term STEMI — and — reviews the basics on how to predict the "culprit" artery.

ECG Media PEARL #11 (6 minutes Audio) — Reviews how to tell IF the “culprit” (ie, acutely occluded) artery has reperfused, using clinical and ECG criteria.


Related ECG Blog Posts to Today’s Case:
  • ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.

  • ECG Blog #193 — illustrates use of the Mirror Test to facilitate recognition of acute Posterior MI. This blog post reviews the basics for predicting the "Culprit" Artery (as well as reviewing why the term "STEMI" — should replaced by "OMI" = Occlusion-based MI).

  • ECG Blog #285 — for another example of acute Posterior MI (with positive Mirror Test).
  • ECG Blog #246 — for another example of acute Posterior MI (with positive Mirror Test).
  • ECG Blog #80 — reviews prediction of the "culprit" artery (and provides another case illustrating the Mirror Test for diagnosis of acute Posterior MI).
  • ECG Blog #317 — reviews another case regarding use (or not) of Posterior Leads.

  • ECG Blog #184 — illustrates the "magical" mirror-image opposite relationship with acute ischemia between lead III and lead aVL (featured in Audio Pearl #2 in this blog post)
  • ECG Blog #167 — another case of the "magical" mirror-image opposite relationship between lead III and lead aVL that confirmed acute OMI.
  • The February 16, 2019 post in Dr. Smith's ECG Blog — My Comment (at the bottom of the page) emphasizes utility of the Mirror Test for diagnosis of acute Posterior MI. 
  • Diagnosis of an OMI from the initial ECG — Serial tracings with spontaneous reperfusion — then reocclusion! — See My Comment at the bottom of the page in the October 14, 2020 post on Dr. Smith's ECG Blog.
  • Acute OMI that wasn’t accepted by the Attending — See My Comment at the bottom of the page in the November 21, 2020 post on Dr. Smith’s ECG Blog.
  • Another overlooked OMI (Cardiologist limited by STEMI Definition — OMI evident by Mirror Test) — See My Comment at the bottom of the page in the September 21, 2020 post on Dr. Smith’s ECG Blog.
  • Recognizing hyperacute T waves — patterns of leads — an OMI (though not a STEMI) — See My Comment at the bottom of the page in the November 8, 2020 post on Dr. Smith's ECG Blog.

  • ECG Blog #271 — Reviews determination of the ST segment baseline (with discussion of the entity of the entity of diffuse Subendocardial Ischemia).

  • ECG Blog #266 — Reviews distinction between Posterior MI vs deWinter T waves (with anterior terminal T wave positivity reflecting "Reperfusion" T-waves).

  • ECG Blog #258 — How to "Date" an Infarction based on the initial ECG.

  • ECG Blog #294 — Reviews how to tell IF the "culprit" artery has reperfused.
  • ECG Blog #230 — Reviews how to compare Serial ECGs.
  • ECG Blog #115 — Shows how dramatic ST-T changes can occur in as short as an 8-minute period.
  • ECG Blog #268 — Shows an example of reperfusion T waves.

  • ECG Blog #190 — How to diagnose acute RV MI (and use of right-sided leads).


  1. Good morning, Prof. This is really very interesting and new to me....another great post. Your Mirror Image test is what I teach medical students and it serves them so well, I love the step by step chronology and the pearls and the flow of events culminating in the final diagnosis. My students always find it easy to follow your trend of thoughts and they come away mesmerized. I am going through many of your old posts, much to learn.

    1. As always — THANKS for the kind words — :) P.S. I sometimes find it interesting to reread some of my "older posts" (some from more than 10 years ago ... ). I fear that some of what I wrote back then is not quite the way that I view things at present ... = "Live and learn" — so please keep that in mind when you see some of my older posts — :)